Many applications require a radial magnetic field (one pointing between the center and the circumference of a circle, along a radial line). One significant use of a radial magnetic field is in the design of a standard loudspeaker. A radial magnetic field creates a magnetic flux through the voice coil windings and generates a force in response to a current through the voice coil which moves the voice coil and the attached sound surface. Current loudspeakers use standard ring magnets (which generate an axial magnetic field) and channel the magnetic field into a radial direction using ferro-magnetic materials. This channeling weakens the magnetic field and reduces the efficiency of the loudspeaker. An alternative system uses wedge-shaped magnets that are glued together to create a radial magnetic field, but this is a complex process and has limited magnetic field potential.
Loudspeakers can require high power to drive them for several reasons, including the ability to move fast and long distances. Existing systems use a voice coil attached to the sound generator (cone), moving in the magnetic field of a fixed magnet-generated gap. The fixed magnet is of limited strength, so the bulk of the power is generated by passing a high current through the voice coil. This has several negative effects. The coil wires must pass high current, forcing them to be thicker. The larger wire puts less turns within the magnetic field, decreasing the force generated to move the voice coil. The increased wire thickness increases the mass of the voice coil, which increases the momentum and opposes changes in motion, requiring more force to move the coil. Finally, the high power causes heat, which forces design complications. Additionally, the requirement for a high power drive signal increases the complexity and cost of the amplifier that must provide this signal.
Loudspeakers typically use an overhung design, meaning the voice coil is longer than the magnetic field gap it moves through. This is because the length of travel that the voice coil has (its throw) is defined by the length of the overlap between the voice coil and the magnetic field gap it travels in. It is difficult to build long magnetic field gaps that are both strong and have a linear magnetic field through the gap, so the throw is increased by making the voice coil longer. This is inefficient because the voice coil has many wasted turns that are not within the magnetic field at any given time, not generating any force but increasing the coil resistance and the coil weight, both wasting power. There are also non-linearities in the magnetic fields around the ends of the voice coil and the magnetic field gap, both of which can cause distortion. To avoid this, the voice coil must be further lengthened to keep the end zones out of the throw of the voice coil. An underhung design, one where the magnetic gap is longer and the voice coil is short, is more efficient because it allows the voice coil to be lighter. The throw is defined by the length of the magnetic gap.